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Spiders - Survival strategies



Spiders - Survival strategies
Can you tell which is the tropical Green Tree Ant and which is the spider that mimics it? Photo: Densey Clyne.
Spiders use many strategies to protect themselves from their enemies. One of the most amazing of these is called autotomy. This is the spider's ability to self-amputate a leg that has been grabbed by a bird or other predator. Usually the leg breaks off close to the body, at the coxa-trochanter joint. Even more amazingly, juvenile spiders can regenerate their legs - a tiny, segmented leg grows within the coxal stump and appears at the next moult. Other strategies include behavioural ploys, like direct threat displays of warning colours on the spider's body, or escaping a predator by dropping quickly away on a silk dragline and playing dead on the ground.



More devious strategies involve camouflage and mimicry. Looking like something you're not, such as a drop of dung or a dangerous or distasteful animal (mimicry), or simply merging into your background (camouflage) has great survival value, especially by day, in avoiding or deterring predators that would otherwise eat the spider. Some of these disguises also fool prey animals into approaching close enough to be ambushed.

The Wrap-around Spider (Dolophones sp.), spins its orb web at night, but by day wraps itself around a twig and 'disappears'. Other nocturnal orb weavers, like Poltys, sit on a branch or on bark by day, and look like broken off twig bases or buds. Although both of these spiders are very conspicuous on their webs at night, they are well hidden from predators like birds and wasps, which are mostly active by day.

Other spiders, like the bark-coloured Two-tailed Spider (Tama) and the leaf-coloured Green Huntsman, simply have colours that blend well into their backgrounds of bark or foliage. Again, this is especially valuable for spiders which are active during the day, like Tama. The Two-tailed Spider gets its name from its long tail-like spinnerets.

The Two-tailed Spiders are also called Rotating Spiders because when prey, such as an ant, comes near, these spiders burst into activity, running rapidly round and round the ant, surrounding it with a barrier carpet of entangling silk bands from the long spinnerets.

Another example of the superb camouflage that has evolved among the spiders is the number of unrelated spiders that imitate bird droppings on vegetation. Two examples are the Dung Spider (Phrynarachne decipiens) and the Bird Dropping Spider (Celaenia kinbergi)

Looking and behaving like an ant is a useful strategy for many spiders, notably jumping spiders (Myrmarachne) and the remarkable thomisid spider, Amyciaea albomaculata. Bird predators avoid this spider because they see it as a Green Tree Ant, which is a fierce biter and stinger. The spider catches and eats stray ants. The ants are easily caught as they seem to accept the spider as one of them - probably because the spider can mimic the ants' chemical scent signals (chemical mimicry). Having made its catch, the spider drops off on a silk thread so that it can eat its meal in safety.




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